1. Field of the Invention
The present invention relates to an optical pickup device, and more specifically, is suitable for use in an optical pickup device that records/reproduces information on/from a recording medium in which a plurality of recording layers is laminated.
2. Disclosure of Related Art
In recent years, optical discs have been increasing in capacity. High-capacity optical discs can be realized by arranging a plurality of recording layers in one disc. For example, there are commercialized multilayer-type discs such as digital versatile discs (DVDs) and Blu-ray discs with two recording layers on one side. Providing three or more recording layers would further increase capacity of optical discs.
For the use of a disc with a plurality of recording layers, an objective lens is generally designed to focus on an intermediate position between the foremost and innermost recording layers. When laser light is converged on each of the recording layers, a spread angle of the laser light with respect to the objective lens is adjusted to be suitable for the recording layer. This reduces laser light aberration at the convergence on each of the recording layers.
However, when the spread angle of laser light is modified as stated above, a rim intensity of the laser light entered into the objective lens changes accordingly. With such a change in the rim intensity, the laser light varies in spot diameter and optical intensity among the recording layers, which may interfere with stable recording performance. This problem becomes more pronounced with a larger number of laminated recording layers and an increasing distance between the foremost and innermost recording layers.
The foregoing problem associated with the rim intensity can be suppressed by reducing rim intensity differences due to shift of a laser light focal position within the range of the foremost to innermost recording layers.